The Water Cycle
There’s a reason Earth is often called the Blue Planet. Water is nearly everywhere, as liquid, vapor and ice. All this shape-shifting water, whatever form it is in at a given moment, is connected. It constantly moves from one phase and location to another.
That perpetual water motion, powered by heat from the sun, is called the water cycle. The water cycle cleans Earth’s water, replenishes it, and transports it from place to place. Not only does the water cycle connect different phases of water, it also connects every living thing. Without the water cycle, life could not exist.
The main processes within the water cycle are evaporation, transpiration, condensation, precipitation, infiltration, and storage.
Let’s follow the water cycle and see where it takes us.
The water cycle is an endless loop, so it doesn’t make too much difference where we start. Let’s begin with evaporation. Evaporation is the process of turning a liquid into a vapor. In the water cycle, evaporation is the way that air moves from the surface up into the atmosphere.
Heat from the sun causes water to evaporate from the oceans, rivers, lakes, and surfaces like sidewalks and roads. Water vapor is less dense than air, so it rises.
You might not always be able to see or feel the evaporation process when it happens outside, but you have probably noticed the signs of it, like vapor rising when you boil water, or a roof steaming when the sun comes out after a rain.
Besides evaporation, about 10 percent of the water in the atmosphere gets there through the process of transpiration. Transpiration is the process of plants releasing water vapor into the air. If you’ve ever seen a photo of clouds rising from a rainforest, you are seeing transpiration.
Condensation and clouds
Condensation is the reverse of evaporation. It’s the process that turns water vapor back into liquid water. When water vapor cools, it condenses. The warmer the air, the more water vapor it can hold before condensing.
You can see condensation on the inside of a window on a cold day. The water droplets that form on the glass are condensation. That’s because the glass is cooler than the air, so the vapor in the air condenses on the glass.
In the water cycle, water vapor condenses as it rises into cooler air. This condensation creates droplets. If it’s very cold, water may turn into ice crystals instead of droplets. When many water droplets or ice particles come together, they form a cloud.
Clouds do more than just drop water and snow. They also transport water from one place to another on the planet. Cloud movement is driven by winds and air currents which are, in turn, driven by the sun’s heat.
A great deal of evaporation and condensation occur over the oceans. Often, the resulting clouds end up over land, providing essential moisture for plants and animals.
Precipitation – what goes up must come down
Precipitation in the water cycle is water falling from clouds. That precipitation can take the form of rain or snow or hail.
Precipitation happens when small droplets combine inside clouds to make bigger droplets. At some point, the droplets become too big. The air and the wind can’t carry them anymore, so they fall to the ground as the precipitation part of the water cycle.
Infiltration and runoff
When it rains, some water filters down into the earth. This is called infiltration. This water may end up near the surface, making the ground muddy and squishy. Or it might travel deeper underground.
Some rain doesn’t flow down into the ground. Instead, it runs along hard or saturated surfaces until it reaches a lake or stream. This is called runoff. Runoff also happens when glaciers melt and form meltwater streams. Streams and rivers flow toward the ocean, feeding the water cycle.
Naturally, some water gets back into the water cycle fairly quickly. But for a lot of Earth’s water, there is a pause in the action. That’s called storage. In fact, most of Earth’s water is in storage at any given moment. Water might be stored for a long time in lakes, oceans and vast ice sheets. About 1.7% of all the water on the planet, including nearly all the freshwater, is stored in glaciers and ice caps.
Water that ends up underground through infiltration or runoff can be stored in aquifers. Aquifers are pockets or layers of underground soil and rock where large and small gaps are saturated with water, like water in a sponge. That water is called groundwater. Humans get some of their drinking water by drilling wells and pumping up water from underground aquifers. Groundwater does flow, but sometimes so slowly that it might not reach daylight for thousands of years!
Adding it all up
This cycle of water is constantly in motion all around us. As you read this, vapor is rising into the air because of evaporation and transpiration. The vapor is condensing into clouds, which may drop precipitation. That rain or snow fills rivers and lakes, which eventually run into oceans and lakes. From there, water evaporates into the air. And the cycle continues, an endless loop.
Although some of Earth’s 1.39 billion cubic kilometers of water is constantly moving through this water cycle, most it is actually in storage at any given moment. As mentioned above, almost 2 percent of all the water on the planet is stored in glaciers and the ice caps. Freshwater, including groundwater, lakes and rivers, accounts for another 1.7%. Water vapor in the air holds only .001% of our water. The rest – almost 97% of the water on Earth – is saltwater stored in the oceans.
The next time you see a raincloud approaching, you might grab your umbrella. You are about to experience the water cycle firsthand!
Written by Laura McCamy
Edited by Beth Geiger, MS Geology
Illustrated by Renee Barthelemy